Method and apparatus for lightning threat indication

ABSTRACT

The invention describes a method of monitoring of increased risk of lightning on the basis of information about the increase of electric charge of water droplets, which are obtained by measuring the characteristics of scattered EM radiation. The change of these characteristics is related to the electric charge, which the droplets acquire. In order to normalize the optical characteristics as well as the concentration of droplets it is necessary that the measurements are carried out at two suitably selected wavelengths. The wavelengths can be combined, allowing the use of two or more wavelengths.

FIELD OF THE INVENTION

The technical solution concerns the field of applied meteorology, mainlythe use of information about the state of electric charge of waterdroplets to indicate the lightning threat for various applications.

DESCRIPTION OF THE RELATED ART

Currently, methods for analyzing the electric potential can be used toforecast the chance of electromagnetic (EM) discharge during storms.These methods most commonly use technologies based on the evaluation ofradar and thermal data. One of the methods using the radar systemoperates, for example, on the principle of coherent and incoherentcorrelation of parallel and cross-polarized radar echo signals. Theelectric charge causes a distortion of the droplet shape, and thereceived signal depends on the shape of droplets. This method isambiguous as droplet shape also can change due to other factors such aswind shear. The degree of non-sphericity of droplets can be estimated onthe basis of the analysis of the correlation coefficient, andconsequently the potential occurrence of lightning can be indicated.

Other patents related to lightning detection are as follows:U.S. Pat. No. 4,792,806, Lightning position and tracking methodU.S. Pat. No. 5,140,523, Neural network for predicting lightningU.S. Pat. No. 5,621,410, Remote prediction of lightning hazardsU.S. Pat. No. 6,405,134, Method and apparatus for predicting lightningthreats based on radar and temperature dataU.S. Pat. No. 6,586,920, Lightning detectorU.S. Pat. No. 6,828,911, Lightning detection and prediction alarm deviceU.S. Pat. No. 7,069,258, Weather prediction method for forecastingselected eventsU.S. Pat. No. 4,792,806, Lightning position and tracking methodOther solutions focused on lightning threat indication are as follows:

US2011090111 A1 20110421

The solution operates on the basis of recognition and classification offorms of hydrometeors from polarimetric radar measurements: signals withdifferent polarization are used.

JPH09329672 A 19971222

The solution according to this patent uses vertically and horizontallypolarized waves, and on the basis of their change in the received radarecho, the shape of the particles is considered and subsequently, to theprobability of storms.

EP0186402 A2 19860702

This solution uses an antenna for the detection of the E- andH-components of the electric and magnetic field. The extent of stormactivity is evaluated as a function of the magnetic field measured intwo frequencies.

The drawbacks of the prior art solutions are that they are highlydependent on external factors, such as wind shear, that affects themorphology of particles, and thus can affect the interpretation ofmeasured data. Misinterpretation of measured data can lead to a falseindication of a potential threat of discharge, i.e. lightning, thusunnecessarily causing economic losses for example in the operation ofairports.

SUMMARY OF THE INVENTION

The invention, according to the proposed solution, constitutes a methodof monitoring of increased risk of lightning on the basis of informationabout the increase of electric charge of water droplets, which isobtained by measuring the characteristics of the scatteredelectromagnetic (EM) radiation. A change of these characteristics isrelated to the electric charge, which the droplets acquire, and not withthe deformation of droplets, thereby solving to a great extent thedrawbacks of the prior art solutions. In order to normalize the opticalcharacteristics as well as the concentration of droplets, it isnecessary that measurements are carried out at two suitably selectedwavelengths. The suitable wavelengths can be arbitrarily combined, whichexplicitly includes the possibility of using two or more wavelengths.

The method of lightning threat indication is based on the fact that theinformation about the electric charge of water droplets is obtained bymeasuring the signal of backscattering of the electromagnetic radiationat two wavelengths. The operational range of wavelengths is from 1 to 50mm; whereas, the dimension of monitored atmospheric particles is by atleast two orders of magnitude smaller. Thresholds are set on theapparatus for lightning threat indication, then the two signals measuredat two different wavelengths are monitored and compared. If the ratio ofthe two signals remains constant, it is a situation without risk oflightning threat. If the ratio of these two signals is increased by atleast 10%, the situation is assessed as a risk in terms of lightningthreat.

The threshold of the apparatus for lightning threat indication is set sothat two arbitrary wavelengths are selected from the range 1 mm to 50mm, and complex refractive indices of the droplets at both wavelengthsare assessed from the tables. Subsequently, the theoretical differencebetween the signal without electric charge and the signal with electriccharge is calculated. If the difference (N) is less than 10%, otherwavelengths must be selected. If the difference N is greater than 10%,the selected wavelengths are applicable for the apparatus and thepresence of electrically charged droplets will be indicated by a signaldifference at the level of at least 0.75N.

The suitable wavelengths can be arbitrarily combined, which implicitlyincludes the possibility of using two or more wavelengths.

The apparatus, which indicates the lightning threat according to thisinvention, consists of a transmitter, a receiver, a control unit, acomputer system and a data storage unit. In the computer system isbidirectionally connected to the control unit and to the data storageunit, within which, data are processed and calculations are performed,wherein the output information from the computer system is thegeneration and sending of a signal with warning of lightning risk.

The transmitter transmits at two or more different wavelengths. Theelectromagnetic wave is transmitted toward the cloud in which scatteringby individual droplets takes place. The scattered waves propagate in alldirections and the waves that travel back to the transmitter aredetected by the receiver, which is located close to the transmitter. Thecomponents of the signal are separated using an electronic circuit or acomputer program. After obtaining the signal intensity at bothwavelengths, the ratio of these signals is evaluated by a computersystem: if during monitoring of this ratio a change of the value of atleast 0.75N is recorded, the change will be interpreted by the controlunit as a threat risk of lightning and the control unit willautomatically generate and send a signal with a warning of lightningrisk.

With the increase of electric charge of water droplets in the atmospherethe probability of lightning occurrence also increases. As shown in [J.Kla{hacek over (c)}ka and M. Kocifaj, “Scattering of electromagneticwaves by charged spheres and some physical consequences,” J.Quantitative Spectrosc. and Radiative Transfer 106, 170-183 (2007) andJ. Kla{hacek over (c)}ka and Kocifaj M., “On the scattering ofelectromagnetic waves by a charged sphere,” Progress In ElectromagneticsResearch, Vol. 109, 17-35 (2010)], the electric charge affects also theoptical characteristics of light and other electromagnetic (EM)radiation.

Recently published theoretical work [J. Kla{hacek over (c)}ka and M.Kocifaj, “Scattering of electromagnetic waves by charged spheres andsome physical consequences,” J. Quantitative Spectrosc. and RadiativeTransfer 106, 170-183 (2007)] has shown that the electric charge ondroplets can affect the characteristics of the scattered EM signal.Changes of this signal are related to several parameters. First,significant changes in the properties of the signal can be expected onlyin the case of droplets that are much smaller than the wavelength of theincident radiation. Also, the material properties of droplets are veryimportant. Droplets with properties close to dielectrics have a greatereffect on the EM radiation than those having metallic properties.

FIG. 1 shows a plot of the ratio of the backscattering efficiency of acharged spherical droplet Q_(bk) (c) to that of an uncharged dropletQ_(bk) as a function of size parameter x=2πα/λ, where α is the radius ofthe water droplet and λ is the wavelength of incident light. Curves areshown for two different wavelengths of EM radiation, at which the waterdroplets have different refractive indices. Different values of chargeare also considered. What is evident from these curves is that thescattered EM signal is predominantly affected by the electric charge. Inthe case of large droplets, the ratio of backscatter efficiencies isequal to one; thus, it is identical to the asymptotic course, when weobserve no effect of the charge on the scattering. However, it isobvious that the values of parameter x, for which a dependence ofbackscattering on electric charge can be observed, are affected by theamount of electric charge. The region in which the effect of electriccharge becomes apparent is called the transition region. For solidcurves in FIG. 1, this transition region is observed in the range ofx˜0.002 to 0.01.

To highlight the changes during backscattering we have used a thick anda thin curve. The position of the transition region depends on thecomplex refractive index of the droplets and also on the wavelength ofthe incident radiation. In the case of a smaller refractive index anddielectric properties of the droplets, the transition region shiftstowards larger values of the parameter x.

According to Pruppacher and Klett [H. R. Pruppacher, J. D. Klett,Microphysics of Clouds and Precipitation, Kluwer Academic Publishers,ISBN 0-7923-4409-X, pp. 930], the amount of elementary charge that canbe placed onto a water droplet ranges from 1 to 10⁸, depending on thesize of the droplet. The larger the droplet, the greater can be thecharge. In calculations of the scattering from charged droplets, it isimportant to take this effect into consideration.

FIG. 2 shows a plot of the charge as a function of droplet radius. Inthe case of (lightning) discharges, we can expect similar amounts ofcharge.

In FIG. 3, the calculations of backscattering efficiency are presented,wherein the course of values of electric charge according to FIG. 2 hasbeen taken into account. The radius of the particles varied in the rangefrom 0.01 to 100 micrometers and the calculations were performed forthree wavelengths. We have chosen these three discrete wavelengths sothat the refractive index of the droplets increases with the increasingwavelength. At the smallest wavelength, the particles have ratherdielectric properties, and we observe the largest changes in thebackscattering efficiency of small charged droplets. At the largestwavelength, these changes are the smallest, and the particles havemetallic properties.

We designate A(α; λ₁:λ₂) to be the ratio of backscattering efficienciesof charged and uncharged droplets at two wavelengths:

${A\left( {a;\lambda_{1};\lambda_{2}} \right)} = {\frac{\frac{{Q_{bk}(c)}\left( \lambda_{1} \right)}{Q_{bk}\left( \lambda_{1} \right)}}{\frac{{Q_{bk}(c)}\left( \lambda_{2} \right)}{Q_{bk}\left( \lambda_{2} \right)}}.}$

the variable α in the above mentioned relation is the droplet radius.When analyzing the ratio A(α; λ₁:λ₂) as a function of droplet radius, wecan find the transition region that separates the first part dependenton electric charge with values of A>1 from the second part with valuesof A nearly indistinguishable from one. We are interested in the firstregion and the ratio mentioned above is designated as A(λ₁:λ₂).

A(λ₁:λ₂)A(α<5 μm;λ₁:λ₂)

The backscattering intensity measured by the detector is a superpositionof the intensities of the scattered radiation generated by all dropletsvisible within the field of view of the apparatus.

In a first-order approximation of the scattering, the intensity ofdetected radiation for each of the above mentioned wavelengths isdetermined by the single scattering from the droplets, thus, forinstance, the total intensity is

${{I(\lambda)} = {\sum\limits_{i}\; {I_{i}(\lambda)}}},$

where I_(i) is the intensity of backscattered light from the i-thdroplet. Because the droplets are much smaller than the wavelength, thescattering by uncharged droplets can be described using the Rayleighscattering approximation:

${I_{i}^{uncharged} = {{I_{0}\frac{x_{i}^{4}a_{i}^{2}}{r_{i}^{2}}{\frac{m_{i}^{2} - 1}{m_{i}^{2} + 2}}^{2}} = {Q_{{bk},i}I_{0}\frac{a_{i}^{2}}{4r_{i}^{2}}}}},$

where I₀ is the intensity of the incident radiation, and for the i-thdroplet we define: x_(i) as the size parameter, α_(i) as the dropletradius, m_(i) as the complex refractive index, r_(i) as the distancefrom the detector and Q_(bk,i) as the backscattering efficiency. Thesize parameter is defined as x_(i)=2πα_(i)/λ, where λ is the wavelengthof incident radiation.

By measuring the backscattering intensities at two wavelengths we cancalculate the value of the ratio

${{R\left( {\lambda_{1}\text{:}\lambda_{2}} \right)} = {\frac{{I_{0}\left( \lambda_{1} \right)}\Sigma_{i}Q_{{bk},i}\frac{a_{i}^{2}}{r_{i}^{2}}}{{I_{0}\left( \lambda_{2} \right)}\Sigma_{i}Q_{{bk},i}\frac{a_{i}^{2}}{r_{i}^{2}}} = \frac{{I_{0}\left( \lambda_{1} \right)}\Sigma_{i}\frac{a_{i}^{6}}{r_{i}^{2}\lambda_{1}^{2}}{\frac{{m_{i}^{2}\left( \lambda_{1} \right)} - 1}{{m_{i}^{2}\left( \lambda_{1} \right)} + 2}}^{2}}{{I_{0}\left( \lambda_{2} \right)}\Sigma_{i}\frac{a_{i}^{6}}{r_{i}^{2}\lambda_{2}^{2}}{\frac{{m_{i}^{2}\left( \lambda_{2} \right)} - 1}{{m_{i}^{2}\left( \lambda_{2} \right)} + 2}}^{2}}}},$

which is a constant independent of number and size of droplets, as theseare much smaller than the wavelength. When the droplets are charged,this ratio will be

${{R(c)}\left( {\lambda_{1}\text{:}\lambda_{2}} \right)} = \frac{{I_{0}\left( \lambda_{1} \right)}\Sigma_{i}{Q_{{bk},i}(c)}\left( \lambda_{1} \right)\frac{a_{i}^{2}}{r_{i}^{2}}}{{I_{0}\left( \lambda_{2} \right)}\Sigma_{i}{Q_{{bk},i}(c)}\left( \lambda_{2} \right)\frac{a_{i}^{2}}{r_{i}^{2}}}$

When considering cloud particles, whose radii correspond to the firstregion with A>1, we can select such wavelengths, for which there islittle or no dependence on Q_(bk)(c)/Q_(bk); i.e.,

${{c\left( \lambda_{n} \right)} = \frac{{Q_{{bk},i}(c)}\left( \lambda_{n} \right)}{Q_{{bk},i}\left( \lambda_{n} \right)}},$

and so we obtain

$\frac{{R(c)}\left( {\lambda_{1}\text{:}\lambda_{2}} \right)}{R\left( {\lambda_{1}\text{:}\lambda_{2}} \right)} = {\frac{c\left( \lambda_{1} \right)}{c\left( \lambda_{2} \right)} = {{A\left( {\lambda_{1}\text{:}\lambda_{2}} \right)}.}}$

This result has the following meaning: by measuring the backscattersignals of EM radiation at the two wavelengths, it is found that theratio of these two signals remains constant in the case of aconventional aqueous cloud, regardless of size or number of droplets.However, from the moment at which the charging of droplets starts tooccur to the moment at which there is a potential threat of lightning,the ratio of signals of scattered radiation will increase proportionallyto the factor A(λ₁:λ₂).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Ratio of backscattering efficiencies of charged and neutraldroplets as a function of size parameter x.

FIG. 2: Plot of electric charge as a function of droplet radius based onthe work of Pruppacher a Klett [3].

FIG. 3: Ratio of the backscattering efficiencies of charged anduncharged droplets as a function of their radius.

FIG. 4: A(α; 1 mm:5 mm) as a function of droplet radius.

FIG. 5: Possible configuration of the apparatus: a simple configurationof the apparatus with a transmitter and a receiver placed at the samelocation.

FIG. 6: Example of realization of the apparatus for two frequencies (f₁and f₂).

FIG. 7: Example of realization for a microwave generator, in which theswitching between the operational frequencies f₁ and f₂ is controlled bya controller.

FIG. 8: Example of realization for a solution with a circulator.

DETAILED DESCRIPTION OF THE INVENTION

The technology described in the summary of the invention is realized inpractice by means of an apparatus that measures the intensity ofbackscattering at the two wavelengths. FIG. 4 depicts the calculatedratio A(1 mm:5 mm), for instance, as a function of droplet radius forthe case where λ₁=1 mm and λ₂=5 mm. This ratio is of about 1.6 forparticles with radius lower than cca 5 μm. The transition region can beobserved for particles with radii of 5-10 μm. For larger particles, thevalue of the function A asymptotically approaches 1. In the first regionwith droplet radii of less than ˜5 μm, the increase of the signals ratiois of approximately 60%.

Example 1

A simple configuration of the apparatus with a transmitter 1 and areceiver 2 located at the same location is depicted in FIG. 5. Thetransmitter 1 generates EM radiation of two frequencies (or wavelengths)and the generated radiation is directed to the cloud of water droplets.The device can be constructed as radar, in which the source of EMradiation is directed to the clouds. In a traditional configuration, thetransmitter 1 and the receiver 2 are simple devices, shaped as aparabola, or they can be separate devices. The wave will be transmittedtowards the cloud, in which scattering on individual droplets occurs.Scattered waves propagate in all directions. Those that travel backtoward the transmitter 1 are designated as backscattering waves and canbe detected by the receiver 2 which is located close to, or collocatedwith, transmitter 1. The detected signal is a superposition ofcomponents of scattered radiation of both wavelengths.

A part of the radiation that is scattered on the droplets travels backand is registered by the receiver 2. The individual components of theintensities for the two wavelengths are recognizable by the computersystem 4. The ratio of these intensities is monitored by the controlunit 3. The data are stored in the data storage unit 5.

Example 2

The second option is to use several detectors, each of which is tuned sothat it collects only a signal of a specific frequency.

FIG. 6 depicts an example of realization for two frequencies (f₁ andf₂), where two generators, microwave generator (1) 6 and microwavegenerator (2) 7, generate microwave radiation with λ₁=1 mm and λ₂=5 mm.The signals are sent to irradiator (1) 8 and irradiator (2) 9 and theradiation is then emitted to the atmosphere by means of parabolicreflector 10.

Example 3

Another approach should be adopted in the case when the transmitterworks in a pulse mode. Then it is convenient to generate and receive asignal of only one frequency, and subsequently to switch the device toanother frequency mode, and after which the whole process is repeatedcyclically. The receiver will process a rectangular signal, wherein thefrequency will correspond to the frequency of the transmitter.Regardless of how the signal intensities at both wavelengths will beobtained, the ratio of these signals is given by the ratio of bothcomponents. If during the monitoring of the ratio A(λ₁:λ₂, a significantchange is registered, this change will be interpreted as a result ofelectric charge in the clouds, and therefore also as a potential risk oflightning. FIG. 7 depicts the realization for microwave generator 14, inwhich the switching between operational frequencies f₁ and f₂ iscontrolled by controller 11.

Example 4

FIG. 8 shows a solution with circulator 13 which releases a signal onlyin a certain direction, and so enables it to lead the signal to theirradiator 8 (AF1) and also to lead the detected signal from theatmosphere to the detector 12. The transition from the MWsource—generator 14 to the detector 12 is not possible.

INDUSTRIAL APPLICABILITY

Lightning is particularly dangerous during landing and take-off of anaircraft, where it can cause loss of power or a complete failure ofaircraft equipment resulting in catastrophe. To avoid this, airportsclose during increased lightning threats. However, the disablement ofairports is extremely costly and therefore it is economicallyadvantageous for airports to have a device for accurate indication ofoccurrence of dangerous lightning.

FIGURE LEGEND

-   1 transmitter-   2 receiver-   3 control unit-   4 computer system-   5 data storage unit-   6 generator 1-   7 generator 2-   8 irradiator 1-   9 irradiator 2-   10 parabolic reflector-   11 controller-   12 detector-   13 circulator-   14 microwave generator

1. Method of lightning threat indication characterized in that theinformation about the electric charge of the water droplets are obtainedby measuring the signal of backscattering of electromagnetic radiationat two wavelengths, wherein the operational range of wavelengths is from1 to 50 mm, the dimension of the monitored particles is by at least twoorders of magnitude smaller, thresholds are set on the apparatus forlightning threat indication, then two signals at two differentwavelengths are monitored and compared, wherein if the ratio of the twosignals remains constant it is a situation without a risk of lightningthreat, and if the ratio of these two signals is increased by at least10% the situation is assessed as a risk of lightning threat.
 2. Methodof lightning threat indication according to claim 1 characterized inthat the threshold of apparatus for lightning threat indication is setso that two arbitrary wavelengths are selected from the range 1 mm to 50mm, complex refractive indices of the droplets at both wavelengths areassessed from tables, subsequently, the theoretical difference betweenthe signal without electric charge and the signal with electric chargeis calculated, and if the difference N alters by less than 10%, thenother wavelengths must be selected, if the change in the difference N isgreater than 10%, the selected wavelengths are applicable for the givenapparatus and the presence of electrically charged droplets will beindicated by the signal difference at the level of at least 0.75N. 3.Method of lightning threat indication according to claim 1 characterizedin that the selected wavelengths can be combined, allowing the use oftwo or more wavelengths.
 4. Apparatus for lightning threat indicationaccording to claim 1 characterized in that the apparatus consists of atransmitter (1), a receiver (2), a control unit (3), a computer system(4) and a data storage unit (5), and in the computer system (4), that isbidirectionally connected to the control unit (3) and to the datastorage unit (5). Data are processed and calculations are performed,wherein the output information from the control unit (3) is thegeneration and sending of a signal warning of lightning risk. 5.Apparatus for lightning threat indication according to claim 4characterized in that several receivers (2) are used, each of which istuned so that it collects only a signal of a specific frequency. 6.Apparatus for lightning threat indication according to claim 4characterized in that the microwave generator (14) operates in pulsemode, wherein a signal of only one frequency is generated and received,and then the microwave generator (14) is switched to the secondfrequency mode and this process is cyclically repeated.
 7. Apparatus forlightning threat indication according to claim 4 characterized in that acirculator (13) is used between the microwave generator (14) and thedetector (12), which releases a signal only in a certain direction,wherein a transition from the microwave generator (14) to the detector(12) is not possible.
 8. Apparatus for lightning threat indicationaccording to claim 2 characterized in that the apparatus consists of atransmitter (1), a receiver (2), a control unit (3), a computer system(4) and a data storage unit (5), and in the computer system (4), that isbidirectionally connected to the control unit (3) and to the datastorage unit (5). Data are processed and calculations are performed,wherein the output information from the control unit (3) is thegeneration and sending of a signal warning of lightning risk.
 9. Methodof lightning threat indication according to claim 2 characterized inthat the selected wavelengths can be combined, allowing the use of twoor more wavelengths.